Sulfur dioxide cycloaddition

Other interesting three-component cycloadditions are the following Sulfur dioxide and diazo compounds lead to episulfones (equation 75)436—in a special case to 4,5-dihydrothiepine S,S-dioxides437 sulfur dioxide, ketene, and arylimine lead to thiazole derivatives438 (equation 76) sulfur dioxide, quinone, and alkenes lead to benzoxathiane derivatives439 (equation 77). 

As formal a, /i-unsaturated sulfones and sulfoxides, respectively, both thiirene dioxides (19) and thiirene oxides (18) should be capable, in principle, of undergoing cycloaddition reactions with either electron-rich olefins or serving as electrophilic dipolarophiles in 2 + 3 cycloadditions. The ultimate products in such cycloadditions are expected to be a consequence of rearrangements of the initially formed cycloadducts, and/or loss of sulfur dioxide (or sulfur monoxide) following the cycloaddition step, depending on the particular reaction conditions. The relative ease of the cycloaddition should provide some indication concerning the extent of the aromaticity in these systems2. 

Similar cycloadditions between thiirene dioxides and 1,3-dipoles generated in situ give heterocycles which result from either loss of sulfur dioxide or from the three-membered ring opening of the initially formed adduct (e.g. 174). Such cycloadditions with nitrilium imides (173a) and nitrile ylids (173b) are illustrated in equation 69175. 

An interesting cycloheptatriene (182) synthesis has been described using thiophene 1, 1-dioxides (180) and cyclopropenes 181 (equation 121)ns. Concerted [4 + 2]cycloaddition and subsequent cheletropic extrusion of sulfur dioxide are suggested by the second-order kinetics (first in each reactant), and by the large negative activation entropy. 

Since sulfoxides and sulfones are versatile synthetic intermediates, and since in both the thiolene oxide and dioxides the reverse dethionylation" (—SO), and cheletropic extrusion of sulfur dioxide , respectively, readily take place thermally, these cycloadditions are expected to find a useful place in organic synthesis. It should be kept in mind, however, that the retrograde SO-diene reaction and interconversion of the thiolene oxides compete effectively against SO extrusion on heating, and that diene isomerization accompanies the forward reaction (SO -t- diene). 

The synthesis of N-phenyl-1,3,4,6-tetrahydrothieno(3,4-c)-pyrrole-2,2-dioxide (II) was carried out by reacting aniline with 3,4-bis(bromomethyl)-2,5-dihydrothiophene-l,1-dioxide (I). The latter compound was synthesized by the bromination of the cycloaddition product, prepared from 2,3-dimethyl-l,3-butadiene and sulfur dioxide (17). 

The reversible reaction between butadiene and sulfur dioxide, which yields sulfolene, has been known for a long time. Such cycloaddition reactions of sulfur dioxide, and of other sulfur oxides, have been reviewed by Block.49... 

The thiophene ring system can be utilized as a synthetic scaffold for the preparation of nonthiophene materials as the sulfur moiety can be removed by reduction (desulfurization) or extrusion (loss of SO2). The extrusion of sulfur dioxide from 3-sulfolenes (2,5-dihydrothiophene 1,1-dioxides) give dienes (butadienes or o-quinodimethanes) that can be utilized to prepare six-membered rings by cycloaddition chemistry. For example, thermolysis of 3-sulfolene 120 provided tricyclic pyrazole 122 via an intramolecular cycloaddition of the o-quinodimethane 121 that results by extrusion of sulfur dioxide <00JOC5760>. Syntheses of 3-sulfolenes 123 and 124 <00S507> have recently been reported. 

Dialkyl-l,2,4-oxadithiolane-2-5 -oxides (160) have been synthesized from the dihydro thia-diazole (161) via nitrogen extrusion and 1,3-dipolar cycloaddition of the intermediate ylide with sulfur dioxide (Scheme 45) <90BSB265>. The formation and trapping of carbonyl oxides is described... 

Thiophene 1,1-dioxides, unlike most other thiophene derivatives, are dienes and combine with dieneophiles to form adducts that are prone to retro-cycloaddition, extruding sulfur dioxide in the process (Scheme 6.34b). 

Thietane 1,1-dioxide derivatives reportedly produce sulfur dioxide as well as a variety of other fragments. Peaks resulting from retro cycloaddition fragmentation such as CH2S and CH2S02 are never lacking. 

Four-membered ring adducts from 2n + 2n cycloaddition of keten-imines to sulfur dioxide. Isolation of l,2-thiazetidin-3-one 1-oxides. jS-Sultames. Simple method for synthesis of substituted 1,2-thia-zetidin 1,1-dioxide. ... 

The following types of dipolarophiles have been used successfully to synthesize five-membered heterocycles containing three heteroatoms by [3 + 2]-cycloaddition of thiocarbonyl ylides azo compounds, nitroso compounds, sulfur dioxide, and Al-sulfiny-lamines. As was reported by Huisgen and co-workers (91), azodicarboxylates were noted to be superior dipolarophiles in reactions with thiocarbonyl ylides. Differently substituted l,3,4-thiadiazolidine-3,4-dicarboxylates of type 132 have been prepared using aromatic and aliphatic thioketone (5)-methylides (172). Bicyclic products (133) were also obtained using A-phenyl l,2,4-triazoline-3,5-dione (173,174). 

O-Quinodimethanes. A recent simple synthesis of Al") 5(lu)-estratriene-l 7-one (5)2 is based on the fact that on pyrolysis substances such as 1 lose sulfur dioxide with generation of o-quinodimethanes.1 The anion of 1 is generated most satisfactorily with KH (1.1 equivalent) in DMF. at 0°. It can be converted predominately to monoalkylated products, particularly if an excess of the anion is used. Thus reaction of the anion of 1 with 2 results in the diastereoisomers 3 and 4. After deketalization, the corresponding ketones are heated at 210" for 8 hours. The o-quinodimethane (a) is formed and undergoes intramolecular cycloaddition to form 5. 

Sulfolene (2,5-dihydrothiophene 1,1-dioxide, 6) is well known to afford 1,3-butadiene and sulfur dioxide in a retrocheletropic cycloaddition (77MI1). 

Dihydro-4if- 1,2-oxazines are conveniently prepared by the cycloaddition of nitrosoalkenes and alkenes. Thus, 3-nitrosobut-3-en-2-one (142) reacts with trans-stilbene to give (143) (78CC847). Similarly, a-nitrosostyrene combines with cyclopentadiene to yield the oxazine (144) <79JCS(Pi)249). Chloronitrones (145) and alkenes in liquid sulfur dioxide containing silver tetrafluoroborate afford oxazinium salts (146) (77JOC4213). 

We indicated previously that sulfur dioxide (S02) and 1,3-butadiene form a [4 + 1 ] cycloaddition product ... 

Exercise 21-24 a. Sulfur dioxide is an angular molecule that can be represented as having a nonbonding electron pair in an sp2 hybrid orbital and one vacant p orbital on sulfur. Use this formulation to derive a thermally allowed transition state for the reversible 1,4-cycloaddition of S02 to 1,3-butadiene (Section 13-3C). b. The three-membered ring sulfone, shown below, is very unstable and rapidly dissociates to S02 and ethene. This process is used for the synthesis of alkenes by the dissociation of cyclic sulfones (Ramberg-Backlund reaction). Determine whether the transition state for the thermally favorable reaction is conrotatory or disrotatory. 

A number of other photochemical 1,4-cycloadditions leading to the formation of heterocyclic systems have been reported, but in general these are less well investigated. The irradiation of o-quinones in the presence of sulfur dioxide affords, often in good yield, the corresponding 1,3,2-dioxathiole or cyclic sulfate 338 [see, for example, Eq. (91)]. Reaction of phenanthraquinone with triphenylphosphine can be achieved photochemically339 or thermally and the product is thought to have the cyclic structure (317). 

Examples of cycloadditions with more than six electrons are relatively rare detailed mechanistic information is in most cases not available. The stereochemistry of the 6 + 2 cheleotropic sulfur dioxide extrusion shown in Equations... 

Another anomalous cycloaddition is the insertion of a carbene into an alkene. 6-Electron cheletropic reactions (p. 28) are straightforward allowed pericyclic reactions, which we can now classify with the drawings 3.47 for the suprafacial addition of sulfur dioxide to the diene 2.179 and its reverse. Similarly, we can draw 3.48 for the antarafacial addition of sulfur dioxide to the triene 2.180 and its reverse. The new feature here is that one of the orbitals is a lone pair, which is given the letter co to distinguish it from o- and n-bonds, with suprafacial and antarafacial defined by the drawings 3.45 and 3.46, which apply to all sp3 hybrids and p orbitals, filled or unfilled. 

Various cyclooctatetraenes have been also prepared by this sequential [67t+2rt] cycloaddition and sulfur dioxide extrusion, as shown in Scheme 6. 

Clearly, there is a fine balance between [2 + 2]- and [4 + 2]-cycloaddition in the reaction of TCNE with vinylindoles. The cycloaddition of 1-methyl-3-vinyl compound and TCNE in benzene gives mainly the [4 + 2]-product, although a small amount of the [2 + 2]-adduct also appeared to be formed. In liquid sulfur dioxide this [4 + 2]-adduct completely isomerized to the [2+ 2]-product. It is probable that in the reaction between diene 205 and TCNE the transition state for [4 + 2]-cycloaddition is destabilized by the steric requirements of the bulky N-benzyl group. 

This strategy has also been extended to [6 + 2]-cycloaddition [61]. Thus, substituted cyclooctatetraenes can be prepared in two steps employing a Cr-(0)-promoted [67t + 27t] thiepin dioxide-alkyne cycloaddition, followed by photoactivated sulfur dioxide extrusion (Scheme 9.37) [62]. 

Scheme 4.2 Diels-Alder cycloaddition of tetrachlorothiophene dioxide (4) and /V-ethyI maleimide (5) yields a high energy intermediate (6) that decomposes spontaneously by elimination of sulfur dioxide. The initially formed product is subsequently oxidized in situ. Transition states for cycloaddition and chelotropic S02 elimination closely resemble the hexachloronorbornene derivative 3 used as a hapten to elicit antibody 1 E9.

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